The present invention relates to polymeric composites, and in particular to methods for the preparation of conductive polymeric composite foams.
Certain conjugated polymers, for example, polypyrrole, polyaniline and polythiophene, are inherently electrically conductive in their oxidized state. However, those polymers are also insoluble, infusible and relatively brittle, so that their applications have been limited. One approach for imparting the desired mechanical properties to these conductive polymers while retaining their excellent electrical properties is to polymerize the conductive polymer within another polymer host. That may be accomplished by an in situ oxidative polymerization of a monomer such as pyrrole in the presence of the polymer host. This approach has been used with good success to prepare conductive composite materials with conductivities from 10−7 to 102 S/cm and in various forms (e.g., films, fibers and foams). See, for example, M. C. DeJesus, Y. Fu and R. A. Weiss, “Conductive Polymer Blends Prepared by In Situ Polymerization of Pyrrole: A Review”, Polym. Eng. Sci. Volume 37, pp. 1936-1943 (1997). One drawback of preparing conductive composites by the in situ polymerization of pyrrole, however, is volatile organic compounds (VOCs) are used as solvents for incorporating the oxidant and/or the pyrrole. The use of the VOCs presents economic and environmental limitations on the process.
A solventless process for preparing conductive polyurethane foams in which supercritical carbon dioxide (scCO2) was used to dissolve an oxidant and swell the polyurethane foam in order to incorporate the oxidant has been reported by Y. Fu, D. R. Palo, C. Erkey and R. A. Weiss in “Synthesis of Conductive Polypyrrole/polyurethane Foams Via a Supercritical Fluid Process”, Macromolecules, Volume 30, pp. 7611-7613 (1997); and by Y. Fu, D. R. Palo, C. Erkey and R. A. Weiss in “Synthesis of Conductive Polypyrrole/polyurethane Foams Via a Supercritical Fluid Process”, ACS Polymer Preprints, Volume 38, p. 430 (1997). That process requires the synthesis of organo-metal sulfonate and organo-metal carboxylate compounds that exhibit higher solubility in scCO2 than prior oxidants commonly used for the in situ polymerization of pyrrole, e.g., ferric chloride. The development of oxidants compatible with scCO2 for the chemical oxidative polymerization of pyrrole were reported at the national meeting of the American Institute of Chemical Engineers in 1998 (I. Kaya, C. Erkey and R. A. Weiss, “Synthesis of Conductive Polymer Composites with Ferric Salts of Fluoroalkylsulfonic Acids as Oxidants Using Supercritical Fluids”, AIChE National Mtg., Orlando, Fla. 1998) and described in the Master of Science Thesis of Ipek Kaya (Synthesis of Conductive Polymer Blends Using Supercritical Carbon Dioxide, University of Connecticut, 1999). Although the scCO2 process eliminates or reduces the need for organic solvents, it is difficult to obtain products with uniform conductivity, and efficient removal of the redox byproduct of the oxidative polymerization reaction often requires use of organic solvents.
Accordingly, there remains a need in the art for methods for the effective in situ polymerization of pyrroles and other monomers within polymers to produce conductive polymer composites, wherein the methods do not require use of VOCs, and particularly wherein the composites have uniform conductivity throughout the polymer.